In a general sense, this invention relates to materials and devices used in the detection of ionizing radiation. More specifically, it relates to scintillator compositions which are especially useful for detecting gamma-rays and X-rays under a variety of conditions.
Many techniques are available for detecting high-energy radiation. Scintillators are of special interest, in view of their simplicity and accuracy. Thus, scintillator crystals are widely used in detectors for gamma-rays, X-rays, cosmic rays, and particles characterized by an energy level of greater than about 1 keV. From such crystals, it is possible to manufacture detectors, in which the crystal is coupled with a light-detection means, i.e., a photodetector. When photons from a radionuclide source impact the crystal, the crystal emits light. The photodetector produces an electrical signal proportional to the number of light pulses received, and to their intensity. Scintillator crystals are in common use for many applications. Examples include medical imaging equipment, e.g., positron emission tomography (PET) devices; well-logging for the oil and gas industry, and various digital imaging applications.
As those skilled in the art understand, the composition of the scintillator is critical to the performance of the radiation detection equipment. The scintillator must be responsive to X-ray and gamma ray excitation. Moreover, the scintillator should possess a number of characteristics which enhance radiation detection. For example, most scintillator materials must possess high light output, short decay time, reduced afterglow, high “stopping power”, and acceptable energy resolution. (Other properties can also be very significant, depending on how the scintillator is used, as mentioned below).
Those skilled in the art are familiar with all of these properties. In brief, “light output” is the quantity of visible light emitted by the scintillator after being excited by a pulse of the x-ray or gamma ray. High light output is desirable because it enhances the radiation detector's ability to convert the light into an electric pulse. (The size of the pulse usually indicates the amount of radiation energy).
The term “decay time” refers to the time required for the intensity of the light emitted by the scintillator to decrease to a specified fraction of the light intensity at the time when the radiation excitation ceases. For many applications, such as the PET devices, shorter decay times are preferred because they allow efficient coincidence-counting of gamma rays. Consequently, scan times are reduced, and the device can be used more efficiently.
“Stopping power” is the ability of a material to absorb radiation, and is sometimes referred to as the material's “X-ray absorption” or “X-ray attenuation”. Stopping power is directly related to the density of the scintillator material. Scintillator materials which have high stopping power allow little or no radiation to pass through, and this is a distinct advantage in efficiently capturing the radiation.
The “energy resolution” of a radiation detector refers to its ability to distinguish between energy rays (e.g., gamma rays) having very similar energy levels. Energy resolution is usually reported as a percentage value, after measurements are taken at a standard radiation emission energy for a given energy source. Lower energy resolution values are very desirable, because they usually result in a higher quality radiation detector.
A variety of scintillator materials which possess most or all of these properties have been in use over the years. For example, thallium-activated sodium iodide (NaI(Tl)) has been widely employed as a scintillator for decades. Crystals of this type are relatively large and fairly inexpensive. Moreover, NaI(Tl) crystals are characterized by a very high light output.
Examples of other common scintillator materials include bismuth germanate (BGO), cerium-doped gadolinium orthosilicate (GSO), and cerium-doped lutetium orthosilicate (LSO). Each of these materials has some good properties which are very suitable for certain applications.
As those familiar with scintillator technology understand, all of the conventional materials possess one or more deficiencies, along with their attributes. For example, thallium-activated sodium iodide is a very soft, hygroscopic material, readily absorbing oxygen and moisture. Moreover, such a material produces a large and persistent after-glow, which can interfere with the intensity-counting system. Furthermore, the decay time of NaI(Tl), about 230 nanoseconds, is too slow for many applications. The thallium component may also require special handling procedures, in view of health and environmental issues.
BGO, on the other hand, is non-hygroscopic. However, the light yield of this material (15 percent of NaI(Tl)), is too low for many applications. The material also has a slow decay time. Moreover, it has a high refractive index, which results in light loss due to internal reflection.
It is thus clear that new scintillator materials would be very welcome in the art, if they could satisfy the ever-increasing demands for commercial and industrial use. The materials should exhibit excellent light output, as well as relatively fast decay times. They should also possess good energy resolution characteristics, especially in the case of gamma rays. Moreover, the new scintillators should be readily transformable into monocrystalline materials or other transparent solid bodies. Furthermore, they should be capable of being produced efficiently, at reasonable cost and acceptable crystal size. The scintillators should also be compatible with a variety of high-energy radiation detectors. Methods for determining the most favorable conditions under which multiple activator ions cooperatively function in the host matrix of a scintillator composition would also be of considerable interest.